Testosterone administration increases skeletal muscle mass and strength, and decreases fat mass in men, but the mechanisms by which testosterone regulates body composition are poorly understood. In the previous funding cycle, we demonstrated that testosterone and DHT promote the differentiation of adult, multipotent. mesenchymal stem cells into myogenic lineage and inhibit their differentiation into adipogenic lineage through an androgen receptor (AR) mediated mechanism. We also discovered that androgens modulate the differentiation of mesenchymal, multipotent cells by activation of Wnt signaling through the association of liganded AR to cytosolic catenin, which causes the latter to translocate to the nucleus and activate Wnt- target genes that in turn regulate mesenchymal multipotent cell differentiation. In this competing continuation application, our objective is to elucidate downstream signaling mechanisms by which AR: b-catenin interaction regulates mesenchymal stem cell differentiation. Our preliminary data show that testosterone upregulates a number of Wnt-target genes, including follistatin. Follistatin inhibits the activity of a number of TGF-b family members, including myostatin, which inhibit myogenesis. We hypothesize that testosterone modulates myogenic differentiation by activation of follistatin, resulting in the inhibition of TGF-/SMAD signaling. We will utilize both in vitro and in vivo approaches to test these hypotheses. Aim 1 will determine whether follistatin is essential for mediating the effects of testosterone on mesenchymal multipotent cell differentiation. We will determine whether knockdown of follistatin through siRNAs blocks testosterone's action on myogenic differentiation in vitro. We will confirm in vivo whether genetic disruption of follistatin expression in Cre/Lox mice blocks testosterone's effects on muscle mass. Aim 2 will determine whether testosterone inhibits TGF/SMAD signaling by its effects on follistatin. We will investigate whether testosterone blocks SMAD signaling by up regulating SMAD7 and by inhibiting SMAD 3 phosphorylation and signaling. Aim 3 (Translational Aim) will determine whether follistatin, acting downstream of AR and b catenin, will provide greater selectivity, and improve muscle mass and function without affecting prostate growth. These translational studies would unveil the mechanisms by which androgens regulate body composition and provide a rational framework for the proposed application of androgens, SARMs, and follistatin as function-promoting therapies. Follistatin, being downstream of AR and b-catenin, would be a promising function promoting anabolic molecule that would increase skeletal muscle mass and function without affecting prostate growth in older men, and in men with prostate cancer who are receiving androgen deprivation therapy.